1. Field of the Invention
This invention is generally related to communication between computers using a layered architecture and, more specifically, to a system and method for forwarding packets using multi-layer information.
2. Description of the Related Art
Communication between computers has become an important aspect of everyday life in both private and business environments. Computers converse with each other based upon a physical medium for transmitting the messages back and forth, and upon a set of rules implemented by electronic hardware attached to and programs running on the computers. These rules, often called protocols, define the orderly transmission and receipt of messages in a network of connected computers.
A local area network (LAN) is the most basic and simplest network that allows communication between a source computer and destination computer. The LAN can be envisioned as a cloud to which computers (also called end stations or end-nodes) that wish to communicate with one another are attached. At least one network element will connect with all of the end stations in the LAN. An example of a simple network element is the repeater which is a physical layer relay that forwards bits. The repeater may have a number of ports, each end station being attached to one port. The repeater receives bits that may form a packet of data that contains a message from a source end station, and blindly forwards the packet bit-by-bit. The bits are then received by all other end stations in the LAN, including the destination.
A single LAN, however, may be insufficient to meet the requirements of an organization that has many end stations, because of the limited number of physical connections available to and the limited message handling capability of a single repeater. Thus, because of these physical limitations, the repeater-based approach can support only a limited number of end stations over a limited geographical area.
The capability of computer networks, however, has been extended by connecting different subnetworks to form larger networks that contain thousands of endstations communicating with each other. These LANs can in turn be connected to each other to create even larger enterprise networks, including wide area network (WAN) links.
To facilitate communication between subnets in a larger network, more complex electronic hardware and software have been proposed and are currently used in conventional networks. Also, new sets of rules for reliable and orderly communication among those end stations have been defined by various standards based on the principle that the end stations interconnected by suitable network elements define a network hierarchy, where end stations within the same subnet have a common classification. A network is thus said to have a topology which defines the features and hierarchical position of nodes and end stations within the network.
The interconnection of end stations through packet switched networks has traditionally followed a peer-to-peer layered architectural abstract. In such a model, a given layer in a source computer communicates with the same layer of a pier end station (usually the destination) across the network. By attaching a header to the data unit received from a higher layer, a layer provides services to enable the operation of the layer above it. A received packet will typically have several headers that were added to the original payload by the different layers operating at the source.
There are several layer partition schemes in the prior art, such as the Arpanet and the Open Systems Interconnect (OSI) models. The seven layer OSI model used here to describe the invention is a convenient model for mapping the functionality and detailed implementations of other models. Aspects of the Arpanet, however, (now redefined by the Internet Engineering Task Force, or IETF) will also be used in specific implementations of the invention to be discussed below.
The relevant layers for background purposes here are Layer 1 (physical), Layer 2 (data link), and Layer 3 (network), and to a limited extent Layer 4 (transport). A brief summary of the functions associated with these layers follows.
The physical layer transmits unstructured bits of information across a communication link. The repeater is an example of a network element that operates in this layer. The physical layer concerns itself with such issues as the size and shape of connectors, conversion of bits to electrical signals, and bit-level synchronization.
Layer 2 provides for transmission of frames of data and error detection. More importantly, the data link layer as referred to in this invention is typically designed to "bridge," or carry a packet of information across a single hop, i.e., a hop being the journey taken by a packet in going from one node to another. By spending only minimal time processing a received packet before sending the packet to its next destination, the data link layer can forward a packet much faster than the layers above it, which are discussed next. The data link layer provides addressing that may be used to identify a source and a destination between any computers interconnected at or below the data link layer. Examples of Layer 2 bridging protocols include those defined in IEEE 802 such as CSMA/CD, token bus, and token ring (including Fiber Distributed Data Interface, or FDDI).
Similar to Layer 2, Layer 3 also includes the ability to provide addresses of computers that communicate with each other. The network layer, however, also works with topological information about the network hierarchy. The network layer may also be configured to "route" a packet from the source to a destination using the shortest path. Finally, the network layer can control congestion by simply dropping selected packets, which the source might recognize as a request to reduce the packet rate.
Finally, Layer 4, the transport layer, provides an application program such as an electronic mail program with a "port address" which the application can use to interface with Layer 3. A key difference between the transport layer and the lower layers is that a program on the source computer carries a conversation with a similar program on the destination computer, whereas in the lower layers, the protocols are between each computer and its immediate neighbors in the network, where the ultimate source and destination endstations may be separated by a number of intermediate nodes. Examples of Layer 4 and Layer 3 protocols include the Internet suite of protocols such as TCP (Transmission Control Protocol) and IP (Internet Protocol).
Endstations are the ultimate source and destination of a packet, whereas a node refers to an intermediate point between the endstations. A node will typically include a network element which has the capability to receive and forward messages on a packet-by-packet basis.
Generally speaking, the larger and more complex networks typically rely on nodes that have higher layer (Layers 3 and 4) functionalities. A very large network consisting of several smaller subnetworks must typically use a Layer 3 network element known as a router which has knowledge of the topology of the subnetworks.
A router can form and store a topological map of the network around it based upon exchanging information with its neighbors. If a LAN is designed with Layer 3 addressing capability, then routers can be used to forward packets between LANs by taking advantage of the hierarchical routing information available from the endstations. Once a table of endstation addresses and routes has been compiled by the router, packets received by the router can be forwarded after comparing the packet's Layer 3 destination address to an existing and matching entry in the memory.
In comparison to routers, bridges are network elements operating in the data link layer (Layer 2) rather than Layer 3. They have the ability to forward a packet based only on the Layer 2 address of the packet's destination, typically called the medium access control (MAC) address. Generally speaking, bridges do not modify the packets. Bridges forward packets in a flat network having no hierarchy without any cooperation by the endstations.
Hybrid forms of network elements also exist, such as brouters and switches. A brouter is a router which can also perform as a bridge. The term switch refers to a network element which is capable of forwarding packets at high speed with functions implemented in hardwired logic as opposed to a general purpose processor executing instructions. Switches come in many flavors, operating at both Layer 2 and Layer 3.
Having discussed the current technology of networking in general, the limitations of such conventional techniques will now be addressed. With an increasing number of users requiring increased bandwidth from existing networks due to multimedia applications to run on the modern day Internet, modern and future networks must be able to support a very high bandwidth and a large number of users. Furthermore, such networks should be able to support multiple traffic types such as dial voice and video which typically require different bandwidths. Statistical studies show that the network domain, i.e., a group of interconnected LANs, as well as the number of individual endstations connected to each LAN, will grow at a faster rate in the future. Thus, more network bandwidth and more efficient use of resources is needed to meet these requirements.
Building networks using Layer 2 elements such as bridges provides fast packet forwarding between LANs but has no flexibility in traffic isolation, redundant topologies, and end-to-end policies for queuing and access control. For example, although endstations in a subnet can invoke conversations based on either Layer 3 or Layer 2 addressing, the higher layer functionalities are not supported by bridges. As bridges forward packets based on only Layer 2 parsing, they provide simple yet speedy forwarding services. However. the bridge does not support the use of high layer handling directives including queuing, priority, and forwarding constraints between endstations in the same subnet.
A prior art solution to enhancing bridge-like conversations within a subnet relies on a network element that uses a combination of Layer 2 and upper layer headers. In that system, the Layer 3 and Layer 4 information of an initial packet are examined, and a "flow" of packets is predicted and identified using a new Layer 2 entry in the forwarding memory, with a fixed quality of service (QOS). Thereafter, subsequent packets are forwarded at Layer 2 speed (with the fixed QOS) based upon a match of the Layer 2 header with the Layer 2 entry in the forwarding memory. Thus, no entries with Layer 3 and Layer 4 headers are placed in the forwarding memory to identify the flow.
However, consider the scenario where there are two or more programs communicating between the same pair of endstations, such as an electronic mail program and a video conferencing session. If the programs have dissimilar QOS needs, the prior art scheme just presented will not support different QOS characteristics between the same pair of endstations, because the prior art scheme does not consider information in Layer 3 and Layer 4 when forwarding. Thus, there is a need for a network element that is flexible enough to support independent priority requests from applications running on endstations connected to the same subnet.
The latter attributes may be met using Layer 3 elements such as routers. But packet forwarding speed is sacrificed in return for the greater intelligence and decision making capability provided by the router. Therefore, networks are often built using a combination of Layer 2 and Layer 3 elements.
The role of the server has multiplied with browser-based applications that use the Internet, thus leading to increasing variation in traffic distribution. When the role of the server was narrowly limited to a file server, for example, the network was designed with the client and the file server in the same subnet to avoid router bottlenecks. However, more specialized servers like World Wide Web and video servers are typically not on the client's subnet, such that crossing routers is unavoidable. Therefore. the need for packets to traverse routers at higher speeds is crucial. The choice of bridge versus router typically results in a significant trade-off, lower functionality when using bridges, and lower speed when using routers. Furthermore, the service characteristics within a network are no longer homogenous, as the performance of a server becomes location dependent if its traffic patterns involve routers.
Therefore, there is a need for a network element that can handle changing network conditions such as topology and message traffic yet make efficient use of high performance hardware to switch packets based on their Layer 2, Layer 3, and Layer 4 headers. The network element should be able to operate at bridge-like speeds, yet be capable of routing packets across different subnetworks and provide upper layer functionalities such as quality of service.